So I'm wondering, and I'll go so far as to say suggesting as well, if BG III will have New Game Plus.

Gaining Bonuses I think would be a good way to do it. An additional attribute point plus a perk for each origin character you beat the game with.

As an example.
First play through
If you beat the game with a vampire spawn you'd get an attribute point and +10% resistance to fire

Second Playthrough
Play as the wizard and you get an an additional attribute point and you get +10% to your magic shield

Third Playthrough
Play as the dark cleric and get an additional attribute point and +10% to your magic damage or +10% to HP recovery through spells or items.

As a few possibilities. I think this sort of system works best as retaining all your money an items would be lame in my opinion. I find it hard to replay an RPG, especially a long RPG, without retaining some sort of perk for the achievement. So this is something I would very much like to see introduced.